means A fine-grained sedimentary rock formed from compacted layers of mud and clay that splits easily into thin sheets, now famous for the oil and gas locked inside it.

from From Old English 'scealu' (a shell or husk) — the same root that gives us 'scale' and 'shell' — pointing to the rock's habit of flaking into thin, peelable layers. The modern geological sense was reinforced in the 18th–19th centuries, possibly nudged along by the German 'Schale' (a shell or peeling), a cousin from the same Germanic stock. The thread running through all of it is the idea of something that comes off in thin sheets.